1. Field of the Invention
The present invention relates to non-reciprocal circuit elements, and, in particular, to a two-port non-reciprocal circuit element, such as an isolator, for use in a microwave band, a composite electronic component including the element, and a communication apparatus including the element or the composite electronic component.
2. Description of the Related Art
Non-reciprocal circuit elements, such as isolators and circulators, have had a characteristic that they transmit a signal only in a predetermined particular direction but do not transmit the signal in the opposite direction. By using this characteristic, for example, isolators are used in transmitting circuit portions of mobile communication apparatuses, such as cellular phones.
As a non-reciprocal circuit element of the related art, Japanese Unexamined Patent Application Publication No. 2002-299915 discloses a three-port isolator in which center electrodes of an input port are balance-connected, and which can connect to a balanced output circuit without using a balun, or a hybrid. In addition, Japanese Unexamined Patent Application Publication No. 2004-282626 discloses a balanced-input balanced-output two-port isolator of a high isolation equivalent circuit that can be connected to a balanced circuit without using a balance-unbalanced converter.
The three-port isolator as described in Japanese Unexamined Patent Application Publication No. 2002-299915 has a narrow input matching band and requires input/output ports and center electrodes dedicated to terminating resistors. Thus, the three-port isolator has a problem reduced reliability since its circuit is complex and expensive.
In addition, the high-isolation two-port isolator as described in Japanese Unexamined Patent Application Publication No. 2004-282626 is not suitable for a transmitting apparatus because it has a narrow pass band and a large insertion loss, and it generates are large amount of heat. Consequently, this high-isolation two-port isolator has a problem of reduced reliability.